This invention relates to load cells and in particular, to a structural member for a load cell that includes overload protection.
In general, a load cell includes as elements formed in a resilient body: a load receiving end portion, a fixed end portion and a pair of sensing beams generally orthogonally disposed therebetween. The sensing beams have strain gages applied thereto. The application of a load to the receiving end coupled with restraint of the opposing fixed end results in relative deflection of the elements in the parallelogram structure. The electrical strain gages are sensitive to the dimensional changes in the load cell structure and an accurate reading of the applied load is derived by external measurement apparatus.
One of the many advantages of a load cell is the lack of individual mechanical parts that are responsive to applied loads and therefore subject to wear from repeated use. The rendering of repeatable and accurate readings by a load cell relies primarily on the structural integrity of the design and the materials used in the load cell structure. Thus, a unitary design relying on a single structural member is preferred. The application of forces in excess of the rated capacity of a load cell plus any design margin can alter the geometry of the load cell structure thereby introducing errors into subsequent readings. Both accuracy and precision are adversely affected. In cases of excessive applied loads, for example, sudden impact loads, the load cell structure can be damaged to the point where it is no longer suitable for use.
To compensate for the application of excessive loads, overload protection is often incorporated in the load cell. One such method of providing overload protection is disclosed in U.S. Pat. No. 4,467,661 to Somal wherein self-contained protection located within the load cell structure utilizes a cantilever beam in combination with an external overload pin extending through the load cell structure into an oversize hole in the end of the beam. The protection feature disclosed therein has the advantage of being confined within the outline of the load cell so that the overall size of the device is not changed. However, in order to obtain the overload protection, additional manufacturing steps are required to provide the receiving holes for a stepped pin. The stepped pin is inserted through the body of the device into the end of the beam. The diameter of the free end of the pin is less than the diameter of the receiving hole in the free end. The clearance between pin and hole edge determines the degree of overload protection. The dimensional change brought about the application of a load to the load cell is limited by the contact of the pin with the edge of the hole in the beam.
The reliability and sensitivity of this type of overload protection depends in part on the alignment of the machined receiving holes, the fit and dimensions of the stepped pin, the clearance between the pin end and the beam, and the mass of the cantilever beam. The overload protection provided by this device appears to be suitable for large load cells but lacks sensitivity for low capacity load cells.
Accordingly, the present invention is directed to a load cell structure having overload protection incorporated into a single unitary structure without requiring the use of additional parts. The structure provides bidirectional protection of the present load cell against both sudden impact forces and loads in excess of rated capacity. The unique constructional features of the load cell structure enable overload protection to be made available for low capacity load cells without changing the external dimensions of the load cell or interfering with the normal operation thereof.
The present invention is directed to a load cell structure of the type receiving strain gages applied thereto for the measurement of forces. The structure is especially well-adapted for use in connection with low capacity load cells and provides protection against larger than rated loads.
The structure comprises a body of resilient material having a force receiving end and a support end laterally spaced therefrom. Intermediate these ends is an active region which includes first and second transition regions integral with the force receiving and support ends respectively. Upper and lower sensing beams extend between the first and second transition regions to complete the parallelogram configuration of a conventional load cell.
Upper and lower horizontal openings bound the respective upper and lower sensing beams. A sensing beam has at least one strain gage receiving area thereon which is normally adjacent to an expanded portion of the adjacent horizontal opening. The first transition region is provided with a vertical opening that extends generally between the planes containing the midline of the upper and lower horizontal openings. The opposing ends of the vertical opening communicate with the upper and lower horizontal openings through slots. Each slot extends between the vertical opening and the corresponding horizontal opening and has a gap width which determines in part the degree of overload protection provided to the load cell.
The horizontal openings, the vertical opening and the slots form a centrally-located cantilever beam extending from the second transition region to the vertical opening. The upper and lower sensing beams are relatively thin so as to be sensitive to applied loads. As a result, the cantilever beam formed in the central region of the load cell structure is a beam of relatively large mass when compared with combined mass of the sensing beams. By increasing the mass of the beam, the cantilever beam is stiffened accordingly and the overload protection is enhanced. When a load is applied to the load cell, the forces in the load cell structure result in a deflection of the sensing beams. This deflection causes one of the gaps in the slots to begin to narrow with closure thereof occurring at a load level deemed to require protection for the load cell.
At the limit of a stop when the upper or lower gap is closed, the transition region contacts the cantilever beam and the load forces are transferred directly to the cantilever beam from the parallelogram structure. Thus, the cantilever beam and the sensing beams are then acting jointly to prevent permanent deformation of the sensing beams as well as dampening any bounce from a sudden impact load.
The load limit at which the overload protection takes effect is determined primarily by the gap width of the upper and lower slots. The gap width defines the limits of deflection of the parallelogram structure of the load cell in relation to the large mass cantilever beam. In practice, the gap width is formed to initiate protection for loads in excess of 140% of the rated load cell capacity. The width of the upper and lower horizontal openings and the width of the vertical opening in the first transition region are normally made larger than the gap width so as to insure that the gap width is the limiting distance in determining the over load protection imparted to the load cell.
Further features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment thereof when taken in conjunction with the accompanying drawings.